Sink Temperature Research Articles

Thermo-hydraulic performance of connected single-phase natural circulation loops characterized by two different inner diameters

In this paper, an experimental study regarding the natural circulation in connected vertical rectangular loops is presented. Typically, the geometry of the natural circulation loops (NCL) is rectangular or toroidal and the analysis is focused on the thermo-hydraulic behaviour of a single working loop. In this study, a new approach referred to several NCLs simultaneously operated was investigated. In the experimental setup, the lower heated sections of the NCLs are connected. The tests were carried out by varying the internal diameter of the loops to investigate the influence of the mutual interaction between two adjacent NCLs. The data were analyzed by the modified Vijayan's correlation showing a good agreement. A simple relationship is proposed to normalize the data both in terms of inner tube diameter and heat sink temperature. The results obtained with two NCLs characterized by different internal diameters could be considered as the first attempt to try optimizing several connected NCLs.

Method of linear approximation of COP for heat pumps and chillers based on thermodynamic modelling and off-design operation

Often, simple estimates of the coefficient of performance (COP) for heat pumps (HPs) and chillers are used. Depending on the purpose, this may not be sufficient. There are more accurate methods for determining COP, but they may not be used due to complexity, nonlinearity, or limited application of advanced COP estimation methods. Here, we present a new COP approximation method suitable for HPs and chillers. It is based on linear relationships and was developed from a thermodynamic two-stage HP model for design and off-design operation using ammonia as a refrigerant. This approximation method was then applied to a case study investigating the potential of district heating supplied by HPs in Tallinn, Estonia. Groundwater, sewage water, seawater, river water, lake water, and a district cooling return line were explored as potential heat sources. The results show a deviation in COP of less than 1.5% compared to the thermodynamic model. Annual calculations show the applicability of the COP approximation method for calculating hourly COPs at different heat source and heat sink temperatures, as well as changing heat loads, seasonal COP, heat demand ratio, and hourly Lorenz efficiency.

Steady-state modelling of dual-evaporator loop heat pipe

To promote the thermal management of loop heat pipe in aerospace and electronics, a steady-state heat transfer model of the asymmetric dual-evaporator loop heat pipe (DE-LHP) is established, in which the mass flow distribution at the junction of the liquid line is modified. The control of operating temperature between two compensation chambers (CCs), heat sharing characteristics and influence of heat sink and ambient temperature on operating temperature are analyzed. The control of operating temperature is influenced by the CCs position and heat load distribution between two evaporators. Two domination zones, namely the position domination zone and heat load distribution domination zone, are observed when the heat load of the distal evaporator is larger than that of the proximal evaporator. A formula for determining the influencing range of the position domination zone is also proposed. The operating temperature reaches the minimum when the heat loads applied to the two evaporators are equal. The ambient temperature influences the operating temperature at low system heat load while the heat sink temperature prevails at high heat load. The heat sharing is affected by the total heat load and the loop pressure distribution, and proportional to the flow distribution at the junction of vapor line.

Experimental investigation on nanoalloy enhanced layered perovskite PCM tamped in a tapered triangular heat sink for satellite avionics thermal management

Miniaturization of circuitry has inherently led to higher heat dissipation which has to be controlled to avoid temperature fluctuation ensuring smooth functioning of the system where developments in the field of PCM provides flexible solutions for avionic Thermal Management (TM). This article focuses to analyse thermal performance of novel tapered triangular finned heat sink along with Vacuum Insulation Panels (VIPs) tamped with nanoalloy (n-LMA) enhanced organo-metallic Mn based layered perovskite Solid-Solid PCM (SS-PCM) to its compatibility for passive TM satellite auxiliary system. An inclusion of 5 wt% nanoalloy (Nano-encapsulated liquid Ga:In eutectic alloy) in SS-PCM increased the thermal conductivity by 21.05% (0.374 W/m-K) and charging and discharging enthalpy by 16.84% (74.56 J/g) and 17.61% (76.32 J/g) respectively. Thermo-physical property values were obtained through Field Emission Scanning Electron Microscope, Scanning Electron Microscope, Differential Scanning Calorimetry and Laser Flash Apparatus. Transient thermal performance analysis was carried out in tapered triangular finned heat sink with specific duty cycles (600s/5400s, 1200s/4800s and 1800s/4200s) for heat fluxes (2 W, 4 W, 6 W, 8 W and 10 W) and with/without VIP for different aspects of SS-PCM and SS-PCM/n-LMA. The effect of n-LMA influences the enhancement in thermal storage and heat transfer performance attributes with a maximum temperature drop of 6.98% over pure SS-PCM. Experimental study reveals better temperature reduction with a maximum of 53.81% decrement in overall heat sink temperature and VIP manifests unidirectional heat transfer through the heat sink reducing overall spatial temperature variation. Further, numerical validation was performed for obtained results with COMSOL application, demonstrating the suitability for real-time passive TM in satellite avionics space applications.

A quasi-online condition monitoring technique for the wind power converter

The power converter plays an important role in the energy conversion and management of wind turbine system. It is one of the core components in the system, but also one of the most prone to fail components according to industry-wide survey. Condition monitoring of power converter can improve the reliable operation of wind turbine system and reduce its unscheduled downtime. In this paper, a novel quasi-online monitoring technique based on the time-constants of heat sink temperature is proposed to check the health status of wind power converter. Advantages of the proposed technique include: (1) This approach enables the monitoring of power module degradation and cooling system deterioration in the converter simultaneously. (2) This method does not need to measure the junction temperature and power loss which reduces the cost and complexity of operation. Experimental tests were conducted to verify the effectiveness of this technique. Results also indicate that the maximum coefficient of variation of time-constants is below 1.7% under different converter operating conditions and ambient temperatures. In addition, the maximum error of time-constants in the application of condition monitoring is less than 3.1%.

Performance assessment of a novel power generation system

This paper introduces a novel power generation system using solar energy as a heat source. The proposed cycle incorporates heat sources from two solar collectors for the effective utilisation of heat energy. To aid the performance of the proposed system, the turbine flow rate is increased with the specific heater arrangements. Energy and exergy balances of the novel system were generated using Python software. The investigation of the present system was evaluated with high sink temperature. Turbine inlet concentration, turbine inlet pressure, HE4 outlet temperature from the turbine, condenser concentration of ammonia, isentropic efficiency of the turbine and pressure ratio are the design variables considered for the exergy and thermoeconomic investigation. The energy and exergy analyses resulted in suitable design variables to optimise the performance. The optimum Kalina cycle efficiency, solar plant efficiency, exergy efficiency and network output were determined to be 18.51%, 8.28%, 34.51% and 295.24 kW, respectively. Among the components involved in the system, the mixers account for the highest exergy destruction followed by the turbine. The cycle performance can be improved by reducing the exergy destruction rate. The thermal efficiency is maximised by the turbine inlet pressure and temperature. Moreover, a higher relative cost difference has resulted in heat exchanger 5 and pump 2.

The Low-Temperature Adsorption Characteristics of Activated Carbon With 3He and 4He as Sorption Cooler Cryogens

Abstract As a commonly used sub-Kelvin refrigeration technology, helium sorption coolers play an important role in space and ground applications. The adsorption characteristics of the porous material inside the sorption cooler at low temperature have a crucial influence on its performance. At present, the analysis and calculation of sorption coolers are mainly based on helium 4 (4He) as the working gas, and there is a lack of systematic research on the low-temperature adsorption characteristics of helium-3 (3He) and its coupling effect characteristics of temperature, pressure, and mass distribution in different components. In this paper, a molecular model of activated carbon that is similar to the actual structure was constructed, and the adsorption isobars and isosteric heat of 3He and 4He at 0.8–5 K were comparatively studied based on the grand canonical Monte Carlo (GCMC) method. Besides, the influence of adsorption characteristics of 3He and 4He on the condensation efficiency, the mass distribution after condensation equilibrium, and the self-cooling loss of the sorption cooler were analyzed. The results show that for the 3He sorption cooler, the main factor affecting the condensation efficiency is the adsorbed helium in the sorption pump, while for the 4He sorption cooler, it is the adsorbed helium and the gas in the dead volume. For both 3He and 4He sorption coolers, the condensation efficiency increases as the sorption pump temperature increases or the heat sink temperature decreases, while the self-cooling loss decreases as the heat sink temperature decreases or the operating temperature increases.

HVDC Converter Cooling System with a Phase Change Dispersion

High voltage direct current converters require efficient cooling of thyristors via heat sinks. Currently, infrastructures use deionised water as a means of cooling the high voltage direct current converters; however, recent research has shown that other fluids have potential to offer more efficient cooling. Phase change dispersions are a new class of heat transfer fluids that employ the latent heat of phase change, thus offering isothermal cooling during melting. For cooling applications, the temperature increase during operation is thus lowered when using phase change dispersions (compared to water) and consequently, the heat sink and thyristors surface temperatures are reduced. In this investigation, a phase change dispersion with non-conductive components, high stability, high capacity and low viscosity has been developed and tested. An experimental setup of a real size heat sink has been installed and the heat transfer behaviour of both the formulated phase change dispersion and water have been investigated and a comparison has been presented. Using water as the heat transfer fluid, the temperature increase from inlet to outlet of the heat sink was 4 K and with the formulated phase change dispersion (at the same mass flow rate and heat input) the temperature increase was 2 K. The phase change dispersion caused a 50% reduction in the heat sink surface temperature. Furthermore, the global heat transfer coefficients obtained for the phase change dispersion were found to be independent of the heating input applied, unlike the trend found for water, additionally, the global heat transfer coefficients were found to be similar to those obtained for water at the same mass flow rates and reached a maximum value of 6100 W m2 K−1. Despite this, the pressure drops and viscosities obtained for the phase change dispersion were higher than for water. Overall, the current investigation demonstrates the ability of using a phase change dispersion as a cooling fluid for the cooling of electronic components, which thus far is limited to using air and water cooling and cannot reach the cooling capacity achieved by phase change dispersions.

Analysis of a Dual Recuperated Dual Expansion Supercritical CO2 Cycle for Waste Heat Recovery Applications

Superior performance and compactness of supercritical CO2 power cycles are encouraging researchers to explore them for waste heat recovery (WHR) applications. This paper presents a comprehensive thermodynamic analysis of a dual recuperated dual expansion cycle utilizing industrial waste heat. The proposed cycle incorporates two recuperators and two turbines with a single compressor. The influence of operating parameters concerning cycle performance in the context of a WHR cycle is discussed. The low side, high side pressures, and the split ratio between the high-temperature and low-temperature turbines are optimized for maximum power rather than thermodynamic cycle efficiency. The cycle performance is evaluated with air as the primary heat transfer fluid in the waste heat recovery heat exchanger operating at a maximum source temperature of 500 °C. The sink temperature is assumed to be 40 °C to enable operation in tropical conditions like India. The paper also compares the performance of the proposed WHR cycle with the baseline single recuperated sCO2 cycle operating under similar conditions. The analysis shows that the proposed cycle is able to extract 60% more heat and produce 37% more power than the single recuperated sCO2 cycle. The maximum heat recovery factor for the analyzed WHR cycle is 17.4%, compared to 12.7% for the single recuperated sCO2 cycle.

Exact, analytical heat transfer from longitudinal radiating fins of rectangular profile in a tube/fin ensemble

AbstractIn this technical note, the problem concerning the quantification of heat transfer rates from an array of longitudinal radiating fins of rectangular profile in a tube/fin ensemble to a nonzero temperature sink is investigated. Radiating fins constitute essential elements in the thermal control of spacecrafts and satellites. We consider quasi one‐dimensional heat conduction in the longitudinal radiating fins and neglect radiative exchange between the fins and the tubes carrying a hot fluid. It is demonstrated that the governing nonlinear differential equation of second order with constant coefficients and nonhomogeneous can be solved in exact, analytical implicit form. The pertinent temperature distributions eventually provide the magnitudes of heat transfer rates and fin efficiencies influenced by the radiation–conduction parameter and the sink temperature.

Experimental investigation on the intensity-balanced dual-frequency Nd: GdVO4 microchip laser

We present an intensity-balanced dual-frequency laser (DFL) based on an Nd: GdVO4 microchip crystal. The intensity balance ratio tuning mechanism of the DFL, which is governed by the heat sink temperature Tc of the laser crystal, is experimentally studied. The experimental results indicate that keeping a balanced intensity condition, when the pumping power of the DFL increases, the heat sink temperature Tc of the laser crystal must be lowered to rebalance the DFL signal intensities. The Tc versus pumping power slope is experimentally measured to be −17.95 ° C / W. By fixing the pumping power at 5.6 W, an intensity-balanced microchip DFL with output power up to 103 mW and frequency separation up to 61 GHz is achieved, whose slope efficiency is 9%.

An estimation of the European industrial heat pump market potential

This paper presents an estimation of the European (EU28) industrial heat pump market potential in terms of magnitude, sizing and number of units. This study is carried out in order to provide technology suppliers and manufacturers of industrial heat pumps perspectives for the technology. Potential heat pump applications in the food, paper, chemical and refining sectors are identified considering a maximum sink temperature of 200°C. This is achieved utilising a bottom-up methodology that uses detailed information from individual processes in the aforementioned sectors. Combining individual process data with typical plant capacities provides information on the heating capacities of heat pumps. The data is upscaled to European level, using production statistics relevant to the individual processes analysed. Since the database of processes is generic in nature and not fully covering the whole industrial sector, the results of this analysis provide a conservative estimate of the heat pump market potential. The results show a potential cumulative heating capacity of industrial heat pumps in EU28 of 23.0 GW, consisting of 4174 heat pump units which are able to cover 641 PJ/a of process heat demand. The largest number of heat pump units (%) can be found for heating capacities <10 MW, making up about 50% of the total market cumulative heating capacity. Clearly, there is a large market ahead for industrial heat pump manufacturers and suppliers.

Energy evaluation of a low temperature commercial refrigeration plant working with the new low-GWP blend R468A as drop-in of R404A

This work analyses experimentally the energy behaviour of a centralized commercial refrigeration plant designed to be used with R404A (GWP100 = 3,943) at low temperatures, when it is replaced with the new low-GWP refrigerant blend R468A (GPW100 = 148). The tests have been done at a selected product temperature of -20°C, Class III indoor conditions according to ISO 23953-2:2015 and 20, 30 and 40°C of heat sink temperature. The product has been kept at the requested temperature in whichever conditions tested. Respect to R404A, running with R468A presents an increase in compressor discharge temperature, greater compressión operation time, and a reduction in compressor electric comssumption that yields in a very similar total energy consumption of the refrigeration plant. So it has been proved that, from an operational point of view, R468A can be a R404A drop in replacement fluid. Also, it has been measured energy savings when an internal heat exchanger is placed.

The efficiency of transcritical CO2 cycle near critical point and with high temperature

Efficiency is a key parameter used to assess the quality of operation of power generation systems and devices applied for converting one type of energy to the other. Although, in the end, an investment project is mainly evaluated by economic aspect. Furthermore, many researchers have been investigating the possible types of energy conversion systems and devices applied for power generation and utilizing different types of working fluids. This paper presents the inside into transcritical carbon dioxide (CO2) cycle and the gradients of its efficiency. Transcritical CO2 cycle (TCO2C) here refers to a CO2-based thermal power generation cycle absorbing heat from a heat source (ideally with constant pressure) till the supercritical state is reached. It is followed by an expansion to a sub-critical superheated or even two-phase (wet) state. As alternatives, trilateral flash cycle (TFC) and organic Rankine cycle (ORC) utilizing CO2 are also introduced in this paper. The calculation in this study is computed based on MATLAB integrated with thermophysical properties like CoolProp and REFPROP, the mathematical models of the system are built and calculated with the same heat sink temperature of 224.41 K, and the heat source temperature is varied between 274.41 K and 500 K. At a certain temperature, the obtained result shows that the efficiency of the TCO2C is lower than the efficiency of ORC. Another result proves that the quality of working fluid at the end of the expansion process significantly influences the efficiency of the cycle.

Electronically tunable near-field radiative heat transfer between doped silicon and graphene-covered silicon dioxide

The key to the micro/nanoscale thermal management of precision instruments is flexibly controlling the heat flux in the near-field following the actual demand. In this paper, a near-field radiative thermal switch (NFRTS) made of the n-type doped silicon (D-Si) and graphene-covered silicon dioxide (SiO2) plates is proposed to achieve the active near-field radiative heat transfer modulation. The radiative heat flux, thermal switching factor, and thermal modulation factor are calculated for different graphene chemical potential values from 0 to 1 eV and D-Si doping concentrations at different vacuum gaps. This is achieved by considering the breakdown voltage of the SiO2, and with the fluctuational electrodynamics and fluctuation-dissipation theorem. The SiO2 is also replaced by the silicon carbide (SiC) in the thermal switch to clarify further the effect of the breakdown voltage on the performance of the NFRTS. In combining the graphene chemical potential value and D-Si doping concentration, it is obtained that the optimal thermal switching factor is 93.5% for a doping concentration of 1018 cm−3 at a 10 nm vacuum gap for SiO2 system when the heat source and heat sink temperatures are 400 K and 300 K, respectively. This is mainly due to the combination of the major angular frequency band of the transmission coefficients for bulk D-Si determined by the doping concentration and the surface mode of the graphene-covered SiO2 modulated by altering the graphene chemical potential value via applying the external voltage bias. Results also reveal that the performance of the NFRTS with SiC substrate is significantly affected and weakened by the limited breakdown voltage of SiC, and almost always worse than that of the NFRTS with SiO2 substrate. This work paves a way for designing the active near-field thermal management devices for simple structures.

Optimization of thermoelectric modules for maximum cooling capacity

A theoretical investigation to optimize thermoelectric modules for maximization of cooling capacity is performed using a novel one-dimensional analytic model, in which the thermal resistances of heat exchangers as well as the thermal and electrical contact resistances inherent in the modules are taken into account. New dimensionless parameters are introduced to avoid the difficulties in optimizing actual (dimensional) cooling capacity encountered in previous dimensional analyses. It is shown that there exist an optimum thermoelement length and an optimum electric current which maximize the cooling capacity when the heat source temperature, heat sink temperature, the thermal resistances of heat exchangers and the contact resistances are given. The effects of the heat exchangers’ thermal resistances and the contact resistances on the attainable maximum cooling capacity, COP, the optimum electric current and thermoelement length are shown.

A novel cooling geometry for subsea variable speed drives

Experimental and theoretical analyses are conducted to evaluate the passive cooling performance of a novel geometry for subsea variable speed drives, a common piece of equipment in deep-sea oil exploration. Relying on the sea water as a low-temperature thermal reservoir, the new design forms an enclosed, annular space with centrally located modular boards that compose the power electronics inverter. Buoyancy-induced motion of a dielectric coolant conveys the heat dissipated by the electronic boards to the sea water through the outer and innermost walls of the annular enclosure. A thermal network model is implemented and used to optimize the enclosure geometry through a genetic algorithm, which served as a reference for a scaled experimental setup. A Computational Fluid Dynamics (CFD) simulation of the conjugate heat transfer yielded temperature distributions on the electronic boards and temperature and fluid velocity fields inside the enclosure. A comparison between the experimental data and the modeling results indicated a good agreement, with average RMS deviations of a modified Nusselt number of 7.0% and 8.5% for the thermal network and CFD analysis, respectively. For a 140-W operating point dissipation rate in the scaled test setup, the thermal network and the CFD models presented maximal deviations of 4°C and 2.3°C with respect to the heat sink temperature measurements.

Magnetic, magnetocaloric effect and resistivity behavior of Sb substituted MnAs

Magnetic, magnetocaloric effect and resistivity behavior of phase pure MnAs1−xSbx systems, synthesized by solid state reaction, have been studied for x up to 0.3. The systems display ferromagnetic metal to paramagnetic insulator transition and the transition temperature TC systematically decreases with increase in x. Arrot plot analysis indicate that the magnetic transition is of first order for all the samples though thermal hysteresis about TC is observed only for x up to 0.15. Isothermal magnetization measurements have been performed in four different protocols traversing distinctive thermodynamic paths and the isothermal magnetic entropy changes (ΔS) have been estimated using Maxwell thermodynamic equation. ΔS(T) exhibit similar asymmetric peak behavior for samples in the range 0.1–0.3 about the corresponding TC both in shape and magnitude, where as x = 0.05 sample exhibits sharp peak followed by a plateau for field change ΔH in the range 10–70 kOe. The value of extracted ΔS(T) is independent of the measurement protocol within the estimated error for all the samples. The calculated refrigeration capacity for two considered cases of hot and cold sink temperatures, viz. TC – 25 and TC + 25 K and temperatures at the full width half maximum is found to be same in all the samples for any ΔH in the range 10–70 kOe. Resistivity behavior in ferromagnetic region manifests contribution from electron-magnon scattering following T2 dependence and electron-phonon contribution following Bloch-Grüneisen formalism. Our analysis indicates lattice elongation is responsible for reduction in TC and Debye temperature ΘD. It is surmised that enhanced spin fluctuations are responsible for paramagnetic insulating behavior.

Reliability of UVC LEDs fabricated on AlN/sapphire templates with different threading dislocation densities

The impact of different AlN/sapphire template technologies [i.e., planar, epitaxial lateral overgrown (ELO), and high temperature annealed sputtered ELO] is studied with respect to the operation-induced degradation of 265 nm UVC LEDs. UVC LEDs with identical heterostructures were grown on templates providing different threading dislocation densities in the range of 0.8 × 109 cm−2 to 5.8 × 109 cm−2. A long-term stress experiment was performed on batches of LEDs, which were operated at a direct current of 200 mA corresponding to a current density of 60 A/cm2 and at a heat sink temperature of 20 °C. The UVC LEDs on templates with lower threading dislocation densities were found to provide a higher optical power and to degrade slower during 2000 h of operation. The experiment demonstrates an extrapolated L70 lifetime of more than 10 000 h for the high temperature annealed sputtered ELO technology. The results suggest that degradation is caused by operation-induced activation of defects whose density scales with the dislocation density.

Impact and quantification of various individual thermodynamic improvements for transcritical R744 supermarket refrigeration systems based on advanced exergy analysis

Climate change mitigation actions have led transcritical R744 refrigeration systems to take center stage in supermarket applications worldwide. Various measures aimed at making these solutions highly performing in any climate context, in fact, have been developed in the last few years. However, their benefits have been evaluated and quantified almost uniquely with the aid of conventional tools, e.g. energy and environmental assessments. Therefore, in this work the most power thermodynamic tool for such a purpose, i.e. the advanced exergy analysis, was applied to the state-of-the-art transcritical R744 supermarket refrigeration systems for the first time ever. Three advanced solutions, namely R744 unit with parallel compression and a R744 system using parallel compression and overfed evaporators with and without two-phase ejectors, were selected in addition to two baselines, being conventional and improved basic R744 units. The investigation was based on the outdoor temperature of 40 °C and operating conditions derived from field measurements. The results obtained suggested that the conventional basic R744 unit features total irreversibilities of 194.49 kW and total unavoidable irreversibilities of 81.96 kW, whereas total irreversibilities and total unavoidable irreversibilities respectively amounted to 168.80 kW and 81.44 kW for the improved basic R744 system. As parallel compression was considered, potential reductions from 143.30 kW down to 67.20 kW, i.e. approximately 18% lower in comparison with the conventional basic unit and the improved basic system, were assed. The use of overfed evaporators was found to lead to potential decrements in total irreversibilities down to 63.79 kW, which were around 5% lower in comparison with parallel compression and 22% lower than with the conventional basic unit and the improved basic system. In addition, the outcomes associated with advanced exergy analysis implementation revealed the need to adopt two-phase ejectors as well as more efficient parallel compressors. The first measure would permit decreasing the total irreversibilities potentially from 115.04 kW down to 44.35 kW, resulting in decrement by about 46% compared to conventional and improved basic R744 systems, 34% in relation to the unit with parallel compressors and 30.5% over the solution with parallel compressors and overfed evaporators. Also, if 10% more efficient parallel compressors had been available, total irreversibilities, total unavoidable irreversibilities and total avoidable irreversibilities of the solution with two-phase ejectors would have been further reducible by 9.3%, 2.6% and 13.5%, respectively. It could be concluded that ejector-based transcritical R744 supermarket refrigeration systems possibly with high performing parallel compressors are capable of providing great thermodynamic performance at high sink temperatures. In addition, the vapour ejectors were found to be responsible for 22.9% of the total avoidable inefficiencies. These irreversibilities could be decremented by almost uniquely enhancing the performance of the ejectors themselves. However, unlike the results derived from conventional energy and environmental analyses, the outcomes of the advanced exergy methodology did not reveal a significant contribution on the part of the overfed evaporators to the thermodynamic performance enhancement of transcritical R744 supermarket refrigerating plants.